Technique for correcting the crystallo-graphic orientation angle of crystals by the formation of mesas and double face lapping

ABSTRACT

A technique for accurately correcting the orientation angle of crystal plates utilizes a mesa formed along one edge of at least one major surface of the plate. The length of the mesa along the surface and the height are determined by the amount of angle correction desired. After formation of the mesa, the plate is double face lapped until the mesa is removed at which point the orientation angle has been changed the desired amount and the lapped major surfaces of the plate are parallel. The mesa can be formed by various techniques as described.

United States Patent 1 Miller 1 Apr. 16, 1974 [54] TECHNIQUE FOR CORRECTING 3,123,953 3/1964 Merkl 51/283 CRYSTALLO GRAPHIC ORIENTATION 3,603,039 9/1971 Stahr 51/283 X 3,562,965 2/1971 Lange 51/283 ANGLE OF CRYSTALS BY THE FORMATION OF MESAS AND DOUBLE FACE LAPPING Inventor: Anton Johann Miller, Allentown,

[73] Assignee: Bell Telephone Laboratories,

Incorporated, Murray Hill, NJ.

[22] Filed: Dec. 22, 1972 [21] Appl. No.: 317,515

[52] U.S. Cl 51/283, 51/323 ['51] Int. Cl B24b 1/00 [58] Field of Search .f.. 51/28l R, 281 SF, 283,

[56] References Cited UNITED STATES PATENTS 2,378,243 6/1945 Penberthy.... 51/283 X Primary Examiner-A1 Lawrence Smith Assistant Examiner-Marc R. Davidson Attorney, Agent, or Firm-A. D. Hooper [5 7] ABSTRACT 16 Claims, 18 Drawing Figures PATENIEDAPR 16 1914 3 303; 774

FIG. l

22 32 SECTION OF UPPER LAP FIG. 3A 29 I saw F MSECTION OF 34 LOWER LAP FIG. 3B

PATENTEDAIR is w 3.803; 774

SHEET 2 [IF 6 I FIG. 2 27 o8 INCREASING I RESISTANCE 07' T0 ABRASION I I 03' oll V- I O2| QORI I g 0| AMOUNT OF ANGLE CORRECTION [IO-MINUTES OF ARC PER .OOI INCH FOR SINGLE MESA o a;

AMOUNT OF ANGLE CORRECTION AG-MINUTES OF ARC PER .OOI INCH FOR TWO MESAS LENGTH OF MESA 2| AS A PERCENTAGE OF THE LENGTH OF PLATE I0 PATENTEDAPR 16 m4 SHEET 3 BF 6 O 0 MW 0 0 m 0 0 o O 5 O O o r 3 w c m m A c F R F O A O F S m 0 E E W E 0 I U o U N U N I m m N m m M l 0 5 M 5. 2 I. .l O. mmmwwmwwmwm ma z zoiuwmmou M3024 mo b23053 THICKNESS 24 OF MESA 2i (INCHES) FIG. 6

sum or 6 PATENTEDAPR 16 I974 3 4 e DECREASES TECHNIQUE FOR CORRECTING THE CRYSTALLO-GRAPHIC ORIENTATION ANGLE OF CRYSTALS BY THE FORMATION OF MESAS AND DOUBLE FACE LAPPING BACKGROUND OF THE INVENTION 1. Field of the Invention The invention relates to the manufacture of crystal plates for frequency control apparatus and the like, and more particularly to a technique for accurately correcting the orientation angle of crystal plates.

2. Description of the Prior Art Crystal plates for frequency control apparatus cannot be cut accurately enough to insure economical yields for high precision applications. For example, many applications call for plates having orientation angles within 1 30 seconds are or less of a specified value. However, cutting accuracy is at best not nearly this precise. Thus, after the cutting operation crystal plates for precision applications must have the orientation angles thereof corrected or many of the plates must subsequently be rejected.

Present apparatus and methods for correcting orientation angles of crystal-plates are not adequate to provide the tolerances previously mentioned. Such apparatus usually comprises a diamond grinding wheel and a chuck upon which the plates to be corrected are mounted one at a time. The relative positions of the wheel and the chuck holding the plate being corrected are controlled by micrometers. This apparatus has a capability of correcting the orientation angle of a'crystal plate to an accuracy of approximately :t 2 minutes of are which is not sufficient for many applications. Another disadvantage of this apparatus is that only one plate can be corrected at a time. Still another disadvantage of the apparatus is that the diamond grinding operation leaves an undesired surface condition on the plate which must subsequently be removed.

In view of the foregoing disadvantages of the present apparatus and methods, crystal plates for precision applications are normally carefully selected from production plates. This selection process results in very low yields of usable plates for specific applications and accordingly high costs for the crystal plates. Thus a need remains for a method of correcting the orientation angle of crystal plates to provide economical yields of such plates for precision applications.

Accordingly, it is an object of this invention to improve the methods for correcting the orientation angle of crystal plates to allow simultaneous correction of a plurality of crystal plates. I

Another object is to improve the methods of correcting the orientation angle of crystal plates to provide more precise corrections.

SUMMARY OF THE INVENTION The foregoing objects and others are achieved in accordance with the invention by forming a mesa of material along one edge of a least one major surface of the plate whose orientation angle is to be corrected. The length and height of the mesa are detennined by the amount of angle correction desired. The mesa can be formed by varioustechniques such as etching, machining, or mounting a chip on the surface of the plate; The plate having the mesa thereon is double face lapped until the mesa is removed at which point the orientation angle has been changed the desired amount and the lapped surfaces of the plate are parallel.

BRIEF DESCRIPTION OF THE DRAWING The invention will be more fully comprehended from the following detailed description and accompanying drawing in which:

FIG. 1 is a perspective view of a crystal plate indicating the desired angle correction;

FIG. 2 is a perspective view of a crystal plate having a mesa formed on a major surface;

FIG. 3A-3D are a sectional representation of the lapping steps of the technique subsequent to the formation of the mesa of FIG. 2;

FIG. 4 is a characteristic plot of the amount of angle correction versus the length of the mesa for various materials;

FIG. 5 is a characteristic plot of the amount of angle correction as a function of mesa thickness;

FIG. 6 is a perspective view of a crystal plate having mesas formed on both major surfaces;

FIG. 7 is a sectional representation of the crystal plate of FIG. 6 after the completion of the correction process;

FIG. 8A and 8B are perspective representations of different locations of mesas for obtaining different directions of angle correction for AT-cut crystal plates;

FIG. 9 is a perspective view of one of many possible arrangements for-batch forming of mesas by etching;

FIG. 10 is a perspective view of a crystal plate having a second embodiment of a mesa;

FIG. 11 isa sectional view of a means for batch forming of the mesas of FIG. 10;

FIG. 12 is a sectional view of the method for mounting the crystal plates of FIG. 11 by bonding; and

FIG. 13A and 13B are sectional views of a second means for forming the mesas of FIG. 10.

DETAILED DESCRIPTION FIG. 1 illustrates in general the geometrical aspects of crystallographic angle correction of crystal plates in accordance with this invention. A crystal plate 10 has a first orientation after the major manufacturing step such as the cutting operation. For example, the crystal plate could be an AT-cut plate having a critical dimension in the ZZ' direction as shown. The preciseorientation in the Z2 direction cannot be obtained to the desired tolerance by existing manufacturing techniques. Accordingly, the orientation angle of the crystal plate must be changed by an amount A0 if the plate is to operate satisfactorily in precision applications. This is accomplished by removing a wedge or section 12 of material from a major surface of face 16 of plate 10 so that to a very close approximation A6 S/R where A0 is the desired amount of angle correction, R is the 22' dimension of the plate, i.e., the length of wedge 12, and S is the maximum thickness of section 12 of material removed. Wedge 14 is then removed from surface 17 to make the major surfaces parallel. The thickness S required to provide a final orientation change of A0 can be considered as one angle unit of material. It should be noted that the addition of a section of material to surface 16 of plate 10 having the same configuration as section 12 but rotated therefrom would have the same rotational effect on the orientation angle as the removal of section 12. This fact will be utilized in the technique of this invention in the formation of mesas on plate 10 either by the removal of material from one end or the addition of material to the other end of plate 10 as will be discussed more fully subsequently.

In accordance with the invention the first step in the angle correction technique is the formation of a mesa on at least one major surface of the crystal plate to be corrected. FIG. 2 is a perspective view of one embodiment of a crystal plate 10 having a mesa 21 formed on one major surface 22 thereof. Mesa 21 can be formed by the removal of material from plate 10 in the generally designated area 27 or alternatively can be formed by the addition of a section or chip of material at the location where mesa 21 is desired. Accordingly, the designated boundary 28 between mesa 21 and plate 10 may or may not exist as a distinct physical boundary.

After the formation of mesa 21 on plates 10, plate 10 is mounted within an aperture 29 of a carrier 31 between the upper and lower lapping plates 32 and 33, respectively, of a double face lapping machine as shown in section in FIG. 3A. For example, a planetary four-motion lapping machine well known in the art could be utilized. Plate 10 is subjected to a double face lapping as illustrated in FIGS. 3B-3D until mesa 21 has been remoted from surface 22 at which point the orientation angle has been changed an amount A and surfaces 22 and 34 of plate are parallel and smooth. The change A0 in the orientation angle is produced by the manner in which material is removed from the major surfaces 22 and 34 of plate 10. The force balances between crystal plate 10 and lapping plates 32 and 33 cause wedge-shaped sections 36 and 37 of material to be removed from surfaces 22 and 34, respectively, during the removal of mesa 21 as illustrated in phantom in FIG. 3D. Removal of these wedge-shaped sections 36 and 37 produces the orientation angle change A0. If no mesa 21 had been formed on surface 22, uniformly thick slices or sections of material would have been removed from both surfaces 22 and 34 during the double face lapping and hence no change in the orientation angle 0 would have resulted.

The amount of angle correction provided by mesa 21 is determined by the characteristics of mesa 21 such as the thickness 24, length 25 relative to the length 26 of plate 10, width 23, location of mesa 21 with respect to surface 22, and the material properties of mesa 21. The width 23 is advantageously made the same as the width of plate 10 and mesa 21 is located along one edge of surface 22 so that the amount of angle correction depends upon the other indicated characteristics.

In order to provide an orientation angle change of A0 the mesa 21 utilized in FIG. 2 must have an initial thickness 24 of twice the thickness S discussed with respect to FIG. 1. This can be understood with reference to FIGS. 3A3D by observing that mesa 21 is located on the opposite end of plate 10 from the thick part of section 36. The maximum thickness 38 of section 36 equals thickness S. In essence this means that the formation of mesa 21 initially produces an angle change of twice A0 and the subsequent removal of section 36 produces an angle change in the opposite direction of A0 so that the net angle change remaining after completion of all steps as shown in FIG. 3D is A0 as desired.

The actual value of the orientation angle change provided by mesa 21 having a specified thickness 24 equal to twice thickness S and having a width 23 equal to the width of plate 10 and being located along one edge of a major surface 22 as shown in FIG. 2 is given by:

where A0 is the amount of the angle change or correction;

R is the ratio of the length 25 to length 26; and

K is a constant determined by the ability of the specific mesa material to resist abrasion from lapping.

This equation establishes a parabolic relationship between A0, length 26 and length 25. Characteristic plots of this equation for different types of material are shown in FIG. 4 with the value of A0 specified along the left ordinate of FIG. 4. It can be observed that the maximum amount of angle correction is achieved when mesa 21 covers 50 percent of surface 22, i.e., has a length 25 equal to one-half the length 26 of surface 22.

FIG. 5 shows a characteristic plot 40 of the amount of angle correction A0 as a function of the thickness 24 of mesa 21 where the length 25 of mesa 21 covers 50 percent of length 26 of plate 10. By the use of FIG. 4 in conjunction with FIG. 5 the dimensions required for mesa 21 to provide any required amount of angle correction can be readily determined. The numerical values specified in FIG. 4 and FIG. 5 are merely illustrative and in no way restrict the scope of the invention.

FIG. 6 is a perspective view of a second arrangement of mesas for providing an orientation angle correction for crystal plate 10. In this arrangement two mesas 42 and 44 are respectively formed on surfaces 22 and 34 of plate 10 on opposite ends thereof. Mesas 42 and 44 are advantageouslybut not necessarily identical, i.e., have the same thicknesses 45 and 46, same lengths 47 and 48, and same widths 49 and 50. Plate 10 having mesas 42 and 44 thereon is placed within a lapping machine and subjected to double face lapping as discussed with respect to the single mesa configuration in FIGS. 3A-3D. Wedges 52 and 54 of material are removed from faces 22 and 34, respectively, simultaneous with the removal of mesas 42 and 44 as indicated in phantom in FIG. 7 to change the orientation angle by an amount A0. Wedges 52 and 54 will be identical but oppositely oriented if mesas 42 and 44 are identical and located as shown. Surfaces 22 and 34 are parallel and smooth after the removal of mesas 42 and 44.

If mesas 42 and 44 are identical, the thickness 45 and 46 thereof required to obtain an orientation change of A0 must equal the thickness S discussed with respect to FIG. 1- This is only one-half the thickness 24 required of the mesa configuration of FIG. 2 to obtain the same orientation angle correction A0.

Equation (1) previously discussed also applies to the mesa configuration of FIG. 6 as long as the equation is separately applied to each mesa. The right-hand ordinate in FIG. 4 gives the angle correction A0 as a function of the lengths of mesas 42 and 44 for a thickness S and plot 41 in FIG. 5 gives the angle correction A0 as a function of thickness. It can readily be observed that if the two mesas 42 and 44 are identical to mesa 21, the mesa configuration of FIG. 6 will produce twice the amount of angle correction as that produced by the mesa configuration shown in FIG. 2.

The direction of change in orientation angle 0, i.e., whether the orientation angle increases or decreases, is determined by the location of the mesa or mesas with respect to the crystallographic axes of the crystal plate being corrected. As shown in FIGS. 8A and 8B with reference to the single mesa configuration, the orientation angle of an AT-cut crystal plate, for example, can be made to increase or decrease by shifting the location of mesa 21 from one end of plate to the other end.

Mesas can be formed on crystal plates by various methods as previously mentioned. One such method involves adding a piece of material to a crystal plate. This can be done for example by selective masking and plating or deposition techniques known in the art. A mesa could also be formed by bonding a chip of material of the proper size on the plate at the desired location.

Mesas can also be formed by removing material from a crystal plate on the end opposite where the mesa is desired. This removal of material can be accomplished, for example, by machining or by selective etching. FIG. 9 is a schematic representation of a method for batch formation of mesas by selective etching. Plates 10 can be arranged in layers and rows as shown and selectively covered with masks 60 where mesas are desired. When the masked plates are subsequently exposed to an etching solution a mesa is left on the masked portions. The masks 60 are then removed and plates 10 lapped as previously discussed. I

Although the mesas have been disclosed and shown as having a rectangular parallelepiped configuration there is no requirement of such a configuration. FIG. 10 illustrates variously configured mesas 62, 64 and 66 which can be formed for various amounts of angle corrections. These mesas are formed by'removing tapered sections 68, 70 and 72 of material, respectively, from plate 10 to leave a portion of the original surface 74 which blends with the inclined portion produced by removal of the tapered sections. For example, whentapered section 72 is removed a large plateau or mesa 66 is left.

The mesas of FIG. 10 can be batch formed by lapping or grinding crystal plates being held in a spherically deflected state. For example, as shown in section in FIG. 11 a plurality of plates 10a and 10b can be mounted around the surface of a spherically concave holding plate or correction tool 80. A flat grinding or lapping plate 82 is lowered'to contact plates 10a and 10b and remove wedges 84 and 86 of material therefrom to leave mesas 88 and 90, respectively. The configuration of wedges 84 and 86, and hence the configuration of the respective remaining mesas 88 and 90, depends upon the distance 94 that the plate 10 is mounted from the center line 92 of correction tool 80. For example, plate 10a is mounted further away from center line 92 than is plate 10b. Consequently, section 84 has a greater degree of taper than section 86 and mesa 88 is larger than mesa 90. Thus various types of mesas can be formed in a single batch.

It should be apparent that a fiat correction tool in conjunction with a apherically convex grinding plate, a flat correction tool in conjunction with a spherically concave grinding plate, or a spherically convex correction tool in conjunction with a flat grinding plate could also be utilized as discussed with respect to FIG. 11.

Plates 10 are normally mounted onto correction tool by bonding. In order to accurately control the mesas formed and hence control the angle correction produced thereby it is necessary to be able to accurately control the thickness and uniformity of the bonding layer which bonds plates 10 to correction tool 80. It is further desired that such bonding be done on a batch basis since all other steps in the correction process are adaptable to batch processing.

Accurate control of the bonding layer and batch bonding are obtained as shown in section in FIG. 12 by positioning crystal plates 10 within apertures or openings in a thin flexible carrier 102. For example, carrier 102 can comprise a thin sheet or disc or suitable plastic film, such as that commercially available under the trademark MYLAR, which is held by a retaining ring 103. The carrier 102 containing plates 10 is positioned on correction tool 80 so that each plate 10 lies on a layer of adhesive 104. A pressure plate 106 having a contour complementary to the contour of correction tool 80, i.e., pressure plate 106 is spherically convex to mate with the spherical concave curvature of correction tool 80 in the illustration, is lowered over plates 10 to make contact therewith. Pressure plate 106 advantageously can have a layer 108 of resilient material such as foam rubber on its exterior face to transmit an even force distribution to plates 10. A rotating motion is imparted to either the pressure plate 106 or carrier 102 which causes plate 106 and carrier 102 to rotate together and which together with the pressure applied by pressure plate 106 causes adhesive layer 104 to spread into a thin, highly uniform layer. Accordingly, theaccuracy of the subsequently formed mesas as previously discussed is not affected by variations in the adhesive bonds. This bonding technique can also be utilized in conjunction with flat correction tools or mounting plates and has application in other areassuch as the manufacture of substrates for integrated circuits.

As an alternative to mounting plates 10 directly on correction tool 80 as shown in FIGS. 11 and 12, plates 10 can be mounted on a flexible circular holder 110 as shown in section in FIG. 13A. Holder 110 is mounted over a correction tool 114 and sealed thereto by 0- rings 112 or other appropriate seals. Correction tool 114 has an opening 116 in the bottom thereof which communicates through line 118 which a vacuum pump 120. When a vacuum is applied by pump 120. holder 110 deflects into a spherical contour as shown in FIG. 13B. The exact contour can be controlled by the level of the vacuum pulled by pump 120. While holder 110 is in the spherically deflected configuration, a grinding plate 122 can be lowered into contact with plates 10 to form mesas as previously described. Correction tool 114 can have a variety of contours including that shown by tool 80 in FIGS. 11 and 12.

A large number of crystal plates having different mesas can be double face lapped as discussed with respect to FIG. 3 in a single batch within a lapping ma chine. Accordingly, different amounts of angle change can be readily provided by a single batch. This versatility provides significant advantages over existing angle correction techniques.

It should be noted that the dimensions shown in the figures have not been proportioned. The thickness of the mesas and wedges, the curvature of the grinding plates and correction tools, et cetera, have been exaggerated for clarity.

While the invention has been described with reference to specific embodiments thereof, it is to be understood that various modifications thereto might be made by those skilled in the art without departingfrom its spirit and scope.

I claim:

1. A method for changing the orientation angle of a crystal plate comprising the steps of:

forming a mesa on at least one major surface of said plate; and

double face lapping said plate until said mesa is removed whereby said orientation angle is changed.

2. The method of claim 1 wherein said mesa has a width equal to the width of said major surface and said method comprises changing said orientation angle by an amount determined by:

where: A6 is said amount of change in said orientation angle;

R is the ratio of the length of said mesa to the length of said major surface on which said mesa is formed; and

K is a constant determined by the resistance of said mesa to abrasion.

3. The method of claim 1 wherein said forming step comprises machining away a first portion of said plate on said major surface and leaving a second portion to form said mesa.

4. The method of claim 1 wherein said forming step comprises lapping said major surface with a lapping plate to remove a first portion of said plate and leaving a second portion to form said mesa.

5. The method of claim 4 wherein said lapping step includes the steps of:

mounting said plate on a spherically concave contoured holder;

lapping said crystal plate with a flat lapping plate to remove a wedge of material from said major surface and to leave a portion to form said mesa.

6. The method of claim 5 wherein said mounting step includes the steps of:

applying a layer of adhesive material to said holder;

placing said crystal plate on said adhesive layer with said major surface exposed upwardly,

placing a pressure plate against said crystal plate, said pressure plate having a contour substantially complementary to the contour of said holder; and

applying a pressure to said crystal plate through said pressure plate while rotating said pressure plate whereby excess amounts of said adhesive are squeezed from beneath said crystal plate leaving a thin uniform adhesive layer.

7. The method of claim 4 wherein said lapping step includes the steps of:

mounting said plate on a flexible mounting'plate, said mounting plate being mounted over a cavity within a base member and forming a seal with said base member;

creating a vacuum in said cavity to cause said mounting plate to deflect into said cavity and assume a spherical contour; and

lapping said crystal plate with a flat lapping plate while said mounted plate is deflected to remove a wedge of material from said major surface and leave a portion to form said mesa.

8. The method of claim 1 wherein said forming step comprises etching away a first portion of said plate on said major surface and leaving a second portion to form said mesa.

9. The method of claim 8 wherein said etching step includes the steps of masking said second portion of said major surface to protect said second portion; and

exposing said plate to an etchant to remove said first portion.

10. The method of claim 1 wherein said forming step comprises adding a section of material to said major surface to form said mesa.

11. The method of claim 10 wherein said adding step includes bonding a chip of material to said major surface.

12. The method of claim 1 wherein said mesa has a length equal to one-half the length of said major surface whereby the maximum amount of change in said orientation angle can be obtained.

13. The method of claim 1 wherein said forming step comprises forming a mesa on two major opposed surfaces of said plate.

14. The method of claim 13 wherein said mesas on said two major surfaces are on opposite ends of said plate.

15. The method of claim 1 wherein said lapping step comprises mounting said plate in a planetary, double face lapping machine, and lapping said plate until said mesa is removed.

16. A method of simultaneously changing the orientation angle of a plurality of crystal plates comprising the steps of:

forming a mesa on at least one major surface of each of said plates; and

simultaneously double face lapping said plates until said mesas are removed whereby said orientation angle of each of said plates is changed. 

1. A method for changing the orientation angle of a crystal plate comprising the steps of: forming a mesa on at least one major surface of said plate; and double face lapping said plate until said mesa is removed whereby said orientation angle is changed.
 2. The method of claim 1 wherein said mesa has a width equal to the width of said major surface and said method comprises changing said orientation angle by an amount determined by: Delta theta R(1-R)K where: Delta theta is said amount of change in said orientation angle; R is the ratio of the length of said mesa to the length of said major surface on which said mesa is formed; and K is a constant determined by the resistance of said mesa to abrasion.
 3. The method of claim 1 wherein said forming step comprises machining away a first portion of said plate on said major surface and leaving a second portion to form said mesa.
 4. The method of claim 1 wherein said forming step comprises lapping said major surface with a lapping plate to remove a first portion of said plate and leaving a second portion to form said mesa.
 5. The method of claim 4 wherein said lapping step includes the steps of: mounting said plate on a spherically concave contoured holder; lapping said crystal plate with a flat lapping plate to remove a wedge of material from said major surface and to leave a portion to form said mesa.
 6. The method of claim 5 wherein said mounting step includes the steps of: applying a layer of adhesive material to said holder; placing said crystal plate on said adhesive layer with said major surface exposed upwardly, placing a pressure plate against said crystal plate, said pressure plate having a contour substantially complementary to the contour of said holder; and applying a pressure to said crystal plate through said pressure plate while rotating said pressure plate whereby excess amounts of said adhesive are squeezed from beneath said crystal plate leaving a thin uniform adhesive layer.
 7. The method of claim 4 wherein said lapping step includes the steps of: mounting said plate on a flexible mounting plate, said mounting plate being mounted over a cavity within a base member and forMing a seal with said base member; creating a vacuum in said cavity to cause said mounting plate to deflect into said cavity and assume a spherical contour; and lapping said crystal plate with a flat lapping plate while said mounted plate is deflected to remove a wedge of material from said major surface and leave a portion to form said mesa.
 8. The method of claim 1 wherein said forming step comprises etching away a first portion of said plate on said major surface and leaving a second portion to form said mesa.
 9. The method of claim 8 wherein said etching step includes the steps of masking said second portion of said major surface to protect said second portion; and exposing said plate to an etchant to remove said first portion.
 10. The method of claim 1 wherein said forming step comprises adding a section of material to said major surface to form said mesa.
 11. The method of claim 10 wherein said adding step includes bonding a chip of material to said major surface.
 12. The method of claim 1 wherein said mesa has a length equal to one-half the length of said major surface whereby the maximum amount of change in said orientation angle can be obtained.
 13. The method of claim 1 wherein said forming step comprises forming a mesa on two major opposed surfaces of said plate.
 14. The method of claim 13 wherein said mesas on said two major surfaces are on opposite ends of said plate.
 15. The method of claim 1 wherein said lapping step comprises mounting said plate in a planetary, double face lapping machine, and lapping said plate until said mesa is removed.
 16. A method of simultaneously changing the orientation angle of a plurality of crystal plates comprising the steps of: forming a mesa on at least one major surface of each of said plates; and simultaneously double face lapping said plates until said mesas are removed whereby said orientation angle of each of said plates is changed. 